Cryopumps are used to create exceptionally-low-pressure vacuum conditions by condensing or adsorbing gas molecules onto low-temperature cryopanels cooled by cryogenic refrigerators. Commonly, refrigerators used in this context are designed to perform a Gifford-McMahon cooling cycle. These refrigerators generally include one or two stages, depending upon which gases are sought to be removed from the controlled atmosphere. Two-stage refrigerators are used when removal of low-condensing-temperature gases is desired. The second stage is typically operated at approximately 15 to 20 K to condense gases such as argon, nitrogen and oxygen upon a cryopanel thermally coupled to the second stage.
In contrast, a single-stage cryopump is typically operated between 90 and 120 K. Operating within this temperature range, a single-stage cryopump will effectively remove gases, such as water, which achieve nearly complete condensation at temperatures below 120 K.
One application where single-stage cryopumps have found frequent use is in process tools designed for the manufacture of semiconductors. A diagram of a cluster process tool is provided as FIG. 1. The process tool 100 typically includes a plurality of inter-connected chambers including an entrance load lock 102 and an exit load lock 104. Each of the load locks 102 and 104 includes a pair of slidable doors 106 and 107. An exterior door 106 opens to the outside atmosphere, and an interior door 107 opens to a transfer chamber 108 which serves as the hub of the process tool 100. Process chambers 112, where manufacturing processes such as etching are performed, open to the transfer chamber 108 along its periphery. Within the process tool 100, an arm 110 rotates to transfer elements among the chambers. Each of these chambers is maintained under vacuum.
In a typical operation of the process tool 108, the exterior door 106 of the entrance load lock 102 opens, venting the entrance load lock 102 to a warm rush of air at ambient pressure and temperature. Semiconductor wafers are inserted into the lock 102, and the exterior door 106 is closed. A rough pump non-selectively evacuates the air within the load lock 102 while a cryopump 114 selectively condenses water vapor and other high-condensing-temperature gases. The dual action of these pumps reestablishes vacuum conditions within the load lock 102. When the pressure within the entrance load lock 102 has returned to a sufficiently low level, the interior door 107 opens, and the rotating arm 110 removes the wafers from the load lock 102 and sequentially delivers and retrieves them from each of the processing chambers 112. The ultra-low vacuum within those chambers is maintained by additional vacuum pumps including a two-stage cryopump. Upon completion of processing, the wafers are delivered to the exit load lock 104. Like the entrance load lock 102, the exit load lock 104 is vented when the exterior door 106 is opened to retrieve the wafers; and a rough pump and a cryopump 114 return the load lock 104 to vacuum conditions to prevent an influx of gas into the transfer chamber 108 when the interior door 107 is later reopened for the next transfer of wafers.